Thermally tuned coaxial cable for microwave antennas

ABSTRACT

A coaxial cable, including an inner conductor and an outer conductor surrounding the inner conductor and a thermally responsive material positioned between the outer conductor and the inner conductor. The outer conductor is in a generally concentric relationship to the inner conductor and the inner and outer conductors are adapted to connect to an energy source. A thermal change in the thermally responsive material alters the generally concentric relationship between the outer conductor and the inner conductor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 12/651,762, filed on Jan. 4, 2010, now U.S. Pat.No. 8,258,399, which is a continuation application of U.S. patentapplication Ser. No. 12/351,633, filed on Jan. 9, 2009, now U.S. Pat.No. 7,642,451, which claims the benefit of priority to U.S. ProvisionalPatent Application Ser. No. 61/023,029, titled “THERMALLY TUNED COAXIALCABLE FOR MICROWAVE ANTENNAS” filed Jan. 23, 2008 by Kenlyn Bonn, all ofwhich are incorporated by reference herein.

BACKGROUND

1. Technical Field

The present disclosure relates generally to microwave antennas. Moreparticularly, the present disclosure relates to thermally tuning coaxialcables for microwave antennas.

2. Background of Related Art

Microwave antennas are used in many applications. For example, medicalmicrowave ablation antennas are used by surgeons. In fact, ablationdevices utilizing DC shock, radio frequency (RF) current, ultrasound,microwave, direct heat, or lasers have been introduced and employed tovarious degrees to ablate biological tissues. Ablation devices may beused in open surgical procedures or are sometimes inserted into catheterdevices in order to perform laparoscopic ablation procedures. Thecatheter incorporating the ablation device is generally inserted into amajor vein or artery or through a body cavity. These catheters are thenguided to a targeted location in the body (e.g., organ) by manipulatingthe catheter from the insertion point or the natural body orifice.

During ablation, the dielectric constant of the tissue changes as morewater is boiled off and tissue desiccation occurs. The changing value ofthe dielectric constant alters the antenna's ability to match theoriginally designed impedance of the antenna. In addition, duringmicrowave ablation in tissue, the impedance of the tissue varies duringthe course of ablation. This occurrence directly corresponds to how muchenergy has been deposited into the tissue during the ablation, resultingin temperature increases at the ablation site.

The impedance in the coaxial cable is typically related to theconcentricity of the inner conductor in relationship to the outerconductor. In ablation procedures, however, conventional antenna designsonly allow for an initial impedance match and as ablation occurs, theincrease in mismatch between the tuning point of the antenna and theablated tissue reduces the efficiency of the energy deposition in thetissue.

SUMMARY

The present disclosure relates to a coaxial cable. The coaxial cableincludes an inner conductor and an outer conductor surrounding the innerconductor configured in a generally concentric relationship therewith,the inner and outer conductors adapted to connect to an energy source. Athermally responsive material is positioned between the outer conductorand the inner conductor wherein a thermal change in the thermallyresponsive material alters the generally concentric relationship betweenthe outer conductor and the inner conductor.

The thermally responsive material of the coaxial cable may include firstand second dielectric materials wherein the first dielectric materialhas a first coefficient of thermal expansion and the second dielectricmaterial has a second coefficient of thermal expansion different fromthe first coefficient of thermal expansion.

In another embodiment, the thermally responsive material of the coaxialcable may include a first resistive heating element at least partiallydisposed in the first dielectric material and a second resistive heatingelement at least partially disposed in the second dielectric material. Athermal change may be defined by the application of heat via one or moreof the first and the second resistive heating elements.

In yet another embodiment, the thermally responsive material furtherincludes a first dielectric material that surrounds the inner conductorand a plurality of resistive heating elements disposed in the firstdielectric material and substantially parallel to the inner conductoralong a length of the coaxial cable. A thermal change may be defined bythe application of heat to the first dielectric material via the one ormore of the plurality of resistive heating elements.

In yet another embodiment, the coaxial cable includes a sensor thatmonitors the inner conductor and/or the outer conductor for determininga position of the inner conductor relative to the outer conductor.

In still yet another embodiment, the thermally responsive material ofthe coaxial cable includes a shape memory alloy responsive to changes intemperature and the thermal change in the shape memory alloy alters thegenerally concentric relationship between the outer conductor and theinner conductor.

In another embodiment, the thermally responsive material of the coaxialcable also includes one or more dielectric spacer(s) in alongitudinally-spaced apart relationship with respect to each other.Each of the dielectric spacer(s) includes a coefficient of thermalexpansion wherein the thermal change of the thermally responsivematerial alters the generally concentric relationship between the outerconductor and the inner conductor at each of the plurality of spacers.The coefficient of thermal expansion of each of the plurality of spacersmay not be equal.

The spacers may further include a first spacer with a first dielectricmaterial and a first coefficient of thermal expansion and a secondspacer with a second dielectric material and a second coefficient ofthermal expansion. The first dielectric material and the seconddielectric material may be different materials. The plurality of spacersmay be in a spaced apart relationship with respect to each other.

The present disclosure also relates to a coaxial cable that includes aninner conductor and an outer conductor surrounding the inner conductor,the inner and outer conductors adapted to connect to an energy source. Afirst dielectric material is disposed between the inner conductor andthe outer conductor, the first dielectric material having a first fluidconduit defined therein. A second dielectric material is disposedbetween the inner conductor and the outer conductor, the seconddielectric material having a second fluid conduit defined therein. Thefirst dielectric material and the second dielectric materials areconfigured to position the inner conductor in a generally concentricrelationship relative to the outer conductor and are formed of thermallyresponsive materials wherein a change in temperature of the first or thesecond dielectric material alters the generally concentric relationshipbetween the inner conductor and the outer conductor. Fluid provided tothe first fluid conduit and/or the second fluid conduit defines thechange in temperature and is selectively controllable to regulate thethermal expansion of the thermally responsive material.

The present disclosure also relates to a surgical device including aninner conductor, an outer conductor surrounding the inner conductorconfigured in a generally concentric relationship therewith. An ablativeenergy delivery device is adapted to couple to an ablative energysource, through the inner and outer conductors, to deliver energy totissue. A thermally responsive material is positioned between the outerconductor and the inner conductor wherein a thermal change in thethermally responsive material alters the generally concentricrelationship between the outer conductor and the inner conductor.

The thermally responsive material selectively aligns or misaligns theinner conductor relative to the outer conductor for tuning and impedancematching. The thermally responsive material may position the innerconductor relative to the outer conductor from a first position whereinthe inner conductor is concentrically-aligned with the outer conductorto a second position wherein the inner conductor is notconcentrically-aligned with the outer conductor.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present disclosure are described herein withreference to the drawings wherein:

FIG. 1A is a front, perspective view of a centrally-disposed coaxialcable having an inner conductor held by two materials having differentcoefficient of thermal expansion values, in accordance with anembodiment of the present disclosure;

FIG. 1B is a front, perspective view of an off-center coaxial cablehaving an inner conductor held by two materials having differentcoefficient of thermal expansion values, in accordance with anotherembodiment of the present disclosure;

FIG. 1C is a schematically-illustrated, cross-sectional view of thecoaxial cable of FIG. 1A;

FIG. 1D is a schematically-illustrated, cross-sectional view of thecoaxial cable of FIG. 1B;

FIG. 2A is front, perspective view of an off-centered coaxial cablehaving an inner conductor held by two materials having differentcoefficient of thermal expansion values, in accordance with anotherembodiment of the present disclosure;

FIG. 2B is a front, perspective view of a centrally disposed coaxialcable having an inner conductor held by two materials having differentcoefficient of thermal expansion values, in accordance with anotherembodiment of the present disclosure;

FIG. 2C is a schematically-illustrated, cross-sectional view of thecoaxial cable of FIG. 2B;

FIG. 2D is a schematically-illustrated, cross-sectional view of thecoaxial cable of FIG. 2A;

FIG. 3 is a schematically illustrated, cross-sectional view of a coaxialcable having an inner conductor held by one or more spacers beingcomposed of one or more materials having different coefficient ofthermal expansion values, in accordance with another embodiment of thepresent disclosure;

FIG. 4A is a front, perspective view of an off-centered coaxial cablehaving an inner conductor and a plurality of resistive heating elementsin each of two or more materials having different coefficient of thermalexpansion values, in accordance with another embodiment of the presentdisclosure;

FIG. 4B is a front, perspective view of a centrally disposed coaxialcable having an inner conductor and a plurality of resistive heatingelements in each of two or more materials having different coefficientof thermal expansion values, in accordance with another embodiment ofthe present disclosure;

FIG. 5A is a schematically illustrated cross-sectional view of anoff-centered coaxial cable having an inner conductor and a plurality ofresistive heating elements in one material having one coefficient ofthermal expansion value, in accordance with another embodiment of thepresent disclosure;

FIG. 5B is a front, perspective view of a centrally disposed coaxialcable having an inner conductor and a plurality of resistive heatingelements in one material having one coefficient of thermal expansionvalue, in accordance with another embodiment of the present disclosure;

FIG. 6 is a front, perspective view of a coaxial cable having an innerconductor with a shape memory alloy, in accordance with anotherembodiment of the present disclosure; and

FIG. 7 is a front, perspective view of a centrally-disposed coaxialcable having an inner conductor held by two materials having differentcoefficient of thermal expansion values with a fluid circulatedtherethrough for regulating the thermal expansion of the two materialsin accordance with another embodiment of the present disclosure.

DETAILED DESCRIPTION

To achieve the foregoing and other objects of the present disclosure,methods and devices pertaining to the microwave antennas are disclosed.In general, the present disclosure pertains to a coaxial cable assemblyand, in one embodiment, to a surgical device including the coaxial cableassembly. The surgical device generally includes an ablative energysource and an ablative energy delivery device coupled to the ablativeenergy source. The ablative energy delivery device is configured todeliver ablative energy sufficiently strong enough to cause tissueablation. In most embodiments, the ablative energy is formed fromelectromagnetic energy in the microwave frequency range. Otherapplications are contemplated by the present disclosure, such astelecommunications or other suitable applications in which microwaveantennas are utilized.

Particular embodiments of the present disclosure are describedhereinbelow with reference to the accompanying drawings. In thefollowing description, well-known functions or constructions are notdescribed in detail to avoid obscuring the present disclosure inunnecessary detail. Those skilled in the art will understand that thepresent disclosure may be adapted for use with either an endoscopicinstrument or an open instrument.

While the present disclosure is susceptible to embodiments in manydifferent forms, there is shown in the drawings and will be describedherein in detail one or more embodiments of the present disclosure.However, the present disclosure is to be considered an exemplificationof the principles of the present disclosure, and the embodiment(s)illustrated is/are not intended to limit the spirit and scope of thepresent disclosure and/or the claims herein.

With reference to the drawings, the coaxial cable of the particularembodiments of the present disclosure are shown. The cable may be of anysuitable length, and the figures are not intended to limit the length ofthe cable to a specific length illustrated or any specific length.Instead, only a representative portion or section of cable isillustrated.

Referring to the embodiment of FIGS. 1A and 1B, the coaxial cable 10includes an outer conductor 12, an inner conductor 14, a first material16, a second material 18, a first air gap 20, and a second air gap 22.The inner conductor 14 is connected to an external power source 300.

The coaxial cable 10 may be rigid, rigid-but shapeable or flexible. Thecoaxial cable 10 may be chosen from commercially available standards andis generally designed with a characteristic impedance of 50 Ohms. Inaddition, one side of the coaxial cable 10 may be coupled to a powersupply 300. Also, the other side of the coaxial cable 10 may be coupledto an antenna (not shown) in any suitable manner.

The outer conductor 12 is arranged to be generally concentric withrespect to the inner conductor 14. However, the concentric relationshipmay be configured to meet a particular purpose as explained in moredetail below. Inner conductor 14 is a central conductor used fortransmitting signals and is typically held relative to the outerconductor 12 by first material 16 and second material 18. In oneembodiment, the first material 16 holds the inner conductor 14, whereasthe second material 18 supports the first material 16 without contactingthe inner conductor 14. In other words, only one material contacts theinner conductor 14.

In the illustrated embodiment, the first material 16 and the secondmaterial 18 define first and second air gaps 20, 22 between the innersurface of the outer conductor 12 and the outer surface of the innerconductor 14. The first air gap 20 separates a first portion of thefirst material 16 and a first portion of the second material 18. Thesecond air gap 22 separates a second portion of the first material 16with a second portion of the second material 18.

The inner conductor 14 has a significant effect on the coaxial cable's10 properties, such as the cable's 10 impedance and attenuationcharacteristics. The impedance on the coaxial cable 10 is related to theconcentricity of the inner conductor 14 in relationship to the outerconductor 12. In the first embodiment, a thermal increase to the coaxialcable 10 is used to alter the alignment concentricity of the innerconductor 14 in a manner that would better match a change in tissueimpedance. The coaxial cable 10 in the antenna (not shown) would startwith an initial impedance match to a transmission line interface thatwould gradually taper along the length of the antenna toward a desiredimpedance with either the addition or the subtraction of heat. The tapercould be controlled thermally through additional features, such as acooling jacket or cooling channels.

FIGS. 1A and 1C illustrate the inner conductor 14 in a centered positionwithin the coaxial cable 10. As heat is applied, the inner conductor 14is moved to an off-centered position due to the thermal expansion ofmaterial 18, as shown in FIGS. 1B and 1D. As the tissue impedancechanges, the alignment sensitivity of the cable 10 may be selectivelychanged (e.g., automatically or manually) such that the impedance of thecable 10 better matches the tissue impedance. One or more materials withdifferent coefficients of thermal expansion may be utilized whichmutually cooperate to tune the inner conductor 14 according to a desiredsetting, such as an ohmage setting.

FIGS. 2A and 2D show an off-centered coaxial cable 110 and FIGS. 2B and2C show a centrally disposed coaxial cable 110 having an inner conductorheld by two materials having different coefficient of thermal expansionvalues. The coaxial cable 110 includes an outer conductor 112, an innerconductor 114, a first material 116 and a second material 118. The innerconductor 114 is connected to an external power source 300.

The first material 116 has a first coefficient of thermal expansionvalue and the second material 118 has a second coefficient of thermalexpansion value, the first and second coefficient of thermal expansionvalues being different. During heat transfer, the energy that is storedin the intermolecular bonds between atoms changes. When the storedenergy increases, so does the length of the molecular bond. As a result,materials typically expand in response to heating and contract oncooling. This response to temperature change is expressed as thematerials coefficient of thermal expansion. The coefficient of thermalexpansion is used in two ways: (1) as a volumetric thermal expansioncoefficient and (2) as a linear thermal expansion coefficient.

Therefore, when the temperature applied to the coaxial cable 110changes, the first material 116 expands at a first rate/volume and thesecond material 118 expands at a second rate/volume. Typical materialsused in coaxial cables include variations of PTFE, polyethylene (PE)blends and silica dioxides, however, nearly any thermo-set orthermoplastic with a low dielectric constant can be used in conjunctionwith another material of similar dielectric constant with a differentcoefficient of thermo-expansion. Typically, different polymer grades orblends result in varying material properties so determining the desiredpair of materials would be a result of finding a matching mixture. Theheat generated by the losses in the dielectric material in the cable canalso be utilized to heat material enough to generate the differential inthermal expansion between the varying materials. A variety of differentmaterials with different coefficient of thermal expansion values may beutilized, e.g., ABS Polymer Extruded, ABS Polymer Nylon Blend, PEEKPolyketone, PEKK Polyketone, Nylon PTFE Filled, Polycarbonate Extruded,LDPE (Polyethylene), Polyimide, PTFE Molded, Silica Aerogel andcombinations thereof.

If the first material 116 expands due to a temperature increase, thesecond material 118 contracts due to the differing coefficient ofthermal expansion values of the two materials 116, 118. As a result, asthe ablation zone heats up, the difference in expansion between the twomaterials 116, 118 would cause the inner conductor 114 to changealignment with the outer conductor 112, e.g., move toward a centeredposition as illustrated in FIGS. 2B and 2C.

As can be appreciated, the materials 116, 118 may be designed toselectively (e.g., either automatically or manually) align or misalignthe inner conductor 114 relative to the outer conductor 112 for tuningand impedance matching purposes. In the embodiment, as seen in FIGS. 1Aand 1B, the design could be made to start with the inner conductor 114concentrically centered relative to outer conductor 112 and then movedoff center when the temperature changes. As shown in FIGS. 2A and 2B,inner conductor 114 may be normally off-center relative to outerconductor 112, and as the temperature increases, the inner conductor 114moves toward the concentric center of the coaxial cable 110 when one ofthe materials 116, 118 is heated.

The system described in regard to FIGS. 1A-2B may include anelectrosurgical generator 300 having a microprocessor and sensorcircuitry (not shown) that continually monitors tissue impedance andmeasures offset impedance. The sensor circuitry may also continuallymonitor the position of the inner conductor 114 of a coaxial cable 110with respect to a desired coaxial position (e.g., a center position).The monitor may be operably coupled to a mechanism (shape memory alloy,heat resistive element) as explained in more detail below) forregulating the thermal expansion of at least one of the first and seconddielectric materials 116, 118 to position the inner conductor 114relative to the outer conductor 112 to change the impedance of the innerconductor 114. The microprocessor or the circuitry may also beconfigured to compare the inner conductor positioning to a predeterminedcenter position. If the inner conductor is positioned above or below thepredetermined center position, one or more materials 116, 118surrounding the inner conductor are heated or moved to re-position theinner conductor 114 to a desired position, and the microprocessorreports such findings to a user control or maintains this data forsubsequent use.

FIG. 3 is a schematically illustrated cross-sectional view of a coaxialcable 210 having an inner conductor 214 held by one or more spacers 230,232, 234 being composed of one or more materials having differentcoefficient of thermal expansion values. In FIG. 3, the coaxial cable210 includes an outer conductor 212, an inner conductor 214, a firstmaterial 216, a second material 218, a first spacer 230, a second spacer232 and a third spacer 234. The inner conductor 214 is connected to anexternal power source 300.

The first, second, and third spacers 230, 232, 234 maintain a desiredposition (e.g., a center position) for the inner conductor 214 for atleast a partial length of the coaxial cable 210. Each of the spacers230, 232, 234 may have the same or a different width, and each may becomposed of one material or two or more materials. Also, the materialused for each spacer may be different. For example, a first spacer 230may be composed of a first material 216 and a second material 218,whereas the second and third spacers 232, 234 may be composed of onematerial.

FIG. 4A is a schematically illustrated cross-sectional view of anoff-centered coaxial cable 310 and FIG. 4B is a schematicallyillustrated cross-sectional view of a centrally disposed coaxial cable310 having an inner conductor and a plurality of resistive heatingelements in each of two or more materials having different coefficientof thermal expansion values. In FIGS. 4A and 4B, the coaxial cable 310includes an outer conductor 312, an inner conductor 314, a firstmaterial 316, a second material 318, first resistive heating elements340 and second resistive heating elements 342.

FIG. 4A illustrates the inner conductor 314 in an off-centered positionwithin the coaxial cable 310. As heat is applied via the heatingresistive elements 340, 342 shown in FIG. 4B, the inner conductor 314moves to a centered position due to the thermal expansion of material318. As the tissue impedance changes, the alignment sensitivity of thecable 310 may be selectively changed (e.g., automatically or manually)such that the impedance of the cable 310 better matches the tissueimpedance. One or more materials may be utilized to tune the innerconductor 314 according to a desired setting, such as an ohmage setting.

A plurality of first resistive heating elements 340 may be positioned infirst material 316 and a plurality of second resistive heating elements342 may be positioned in second material 318. The first and secondresistive heating elements 340, 342 convert electricity into heat.Electrical current running through the elements encounter resistance,thus resulting in heating of the element. Resistive heating elements340, 342 may be made from Nichrome which has a relatively highresistance and does not break down or oxidize in air at usefultemperature ranges. First and second resistive heating elements 340, 342may also be positioned in parallel to the inner conductor 314, atvarious lengths from the inner conductor 314, and in various widths. Thetemperature of each of the plurality of heating elements 340, 342 may beselectively controllable to position the inner conductor 314 relative tothe outer conductor 312 and the plurality of heating elements 340, 342may be disposed in a concentric array relative to the inner conductor314.

FIG. 5A is a schematically illustrated cross-sectional view of anoff-centered coaxial cable 410 and FIG. 5B is a schematicallyillustrated cross-sectional view of a centrally disposed coaxial cable410. In FIGS. 5A and 5B, the coaxial cable 410 includes an outerconductor 412, an inner conductor 414, a dielectric material 416, andone or more resistive heating elements 440. In contrast to FIGS. 4A and4B, only one dielectric material 416 is used to surround the entirelength of the inner conductor 414. The dielectric material 416 includesone or more resistive heating elements 440 in parallel to the innerconductor 414 along the length of the cable 410. More particularly, theresistive heating elements 440 are positioned in parallel to the innerconductor 414, at various lengths along the inner conductor 414, and invarious widths.

FIG. 5A illustrates the inner conductor 414 in an off-centered positionwithin the dielectric material 416. As heat is applied, the innerconductor 414 is moved to a desired position (e.g., a center position)due to the thermal expansion of dielectric material 416 and due to firstresistive heating element 440 b being heated to expand the dielectricmaterial 416 in a given direction. Any member or combination of heatingelements 440 a-440 e may be utilized to move the inner conductor 414 fortuning purposes. As the tissue impedance changes, the alignmentsensitivity of the cable 410 may be selectively changed (e.g.,automatically or manually) such that the impedance of the cable 410better matches the tissue impedance.

FIG. 6 is a schematically illustrated cross-sectional view of a coaxialcable having an inner conductor with a shape memory alloy 550 inaccordance with another embodiment of the present disclosure. In FIG. 6,the coaxial cable 510 includes an outer conductor 512, an innerconductor 514, a dielectric material 516 and a shape memory alloy 550.

The shape memory alloy 550 is, for example, positioned in proximity tothe inner conductor 514. One or more shape memory alloys 550 may bepositioned along the length of the coaxial cable 510 in predetermineddistance from each other.

Shape memory alloys (SMAs) are a family of alloys having anthropomorphicqualities of memory and trainability and are particularly well suitedfor use with medical instruments. SMAs have been applied to such itemsas actuators for control systems, steerable catheters and clamps. One ofthe most common SMAs is Nitinol which can retain shape memories for twodifferent physical configurations and changes shape as a function oftemperature. Recently, other SMAs have been developed based on copper,zinc and aluminum and have similar shape memory retaining features.

SMAs undergo a crystalline phase transition upon applied temperatureand/or stress variations. A particularly useful attribute of SMAs isthat after it is deformed by temperature/stress, it can completelyrecover its original shape on being returned to the originaltemperature. The ability of an alloy to possess shape memory is a resultof the fact that the alloy undergoes a reversible transformation from anaustenitic state to a martenistic state with a change intemperature/stress. This transformation is referred to as athermoelastic martenistic transformation.

Under normal conditions, the thermoelastic martenistic transformation.occurs over a temperature range which varies with the composition of thealloy, itself, and the type of thermal-mechanical processing by which itwas manufactured. In other words, the temperature at which a shape is“memorized” by an SMA is a function of the temperature at which themartensite and austenite crystals form in that particular alloy. Forexample, Nitinol alloys can be fabricated so that the shape memoryeffect will occur over a wide range of temperatures, e.g., −2700 to+1000 Celsius. Many SMAs are also known to display stress-inducedmartenisite (SIM) which occurs when the alloy is deformed from itsoriginal austensitic state to a martensitic state by subjecting thealloy to a stress condition.

As a result, when heat is applied to the coaxial cable 510, the innerconductor 514 tends to move from its desired position within the coaxialcable 510. SMA 550, which is embedded within a material 516 having acertain coefficient of thermal expansion and which is located in a closeproximity to the inner conductor 514 may move the inner conductor 514back to its desired position (e.g., a center position) within thecoaxial cable 510. SMA 550 can recover from large amounts of bending andtorsional deformations, due to the application of heat, as well as smallamounts of strain. Provided the deformations are within recoverableranges, the process of deformation and shape recovery can be repeatedmillions of times. As a result, the SMA 550 located within the material516 can repeatedly move the inner conductor 514 back to a desiredposition (e.g., a centered position). Moreover, as can be appreciated,the material 516 may be designed to selectively (e.g., eitherautomatically or manually) align or misalign the inner conductor 514relative to the outer conductor 512 for tuning and impedance matchingpurposes.

Consequently, the embodiments of the present disclosure allow forimproved antenna impedance matching for controlling tissue impedance ofa microwave antenna during an ablation procedure via a thermally tunedcoaxial cable. The embodiments further include changing the impedance ofthe coaxial cable for allowing greater flexibility in designingmicrowave antennas. By having a varying impedance of the coaxial cablein the antenna tuned to change with the increase/decrease intemperature, tissue impedance changes, and thus, the antenna may deposita greater amount of energy over the entire course of the ablationprocedure. By using dielectric cores of varying thermal expansionvalues, it is possible to force the eccentricity of the inner conductorof the coaxial cable on-line or off-line, thus effectively changing thecoaxial cable's impedance value.

In addition, FIGS. 1A-2D illustrate two materials 16, 18 within thespacing between the inner surface of the outer conductor 12 and theouter surface of the inner conductor 14 including two air spaces or gaps20, 22. However, one skilled in the art may use more than two materialswithin the spacing between the inner surface of the outer conductor 12and the outer surface of the inner conductor 14 and more than two airgaps. For example, one skilled in the art may be motivated to use threeor more materials, each with a different coefficient of thermalexpansion value in a triangular configuration with three or more airgaps separating the materials. In addition, one skilled in the art maybe motivated to use two materials in a checkered pattern or any othertype of intertwined pattern with one or more air gaps in order to centeror off-center the inner conductor 14 of the coaxial cable 10 as neededto tune or match the tissue impedance.

Further, in FIGS. 1A-6 there may be one or more mechanisms that regulatethe thermal expansion of at least one of the first and second dielectricmaterials 16, 18 to position the inner conductor 14 relative to theouter conductor 12 to change the impedance of the inner conductor 14.

FIG. 7 shows another embodiment according to the present disclosurewherein the coaxial cable 700 includes an outer conductor 712 arrangedto be generally concentric with respect to the inner conductor 714 usedfor transmitting signals. Inner conductor 714 is held relative to theouter conductor 712 by first material 716 and second material 718 onlyone of which contacts inner conductor 714. First material 716 and thesecond material 718 define first and second air gaps 720 and 722,respectively, between the inner surface of the outer conductor 712 andthe outer surface of the inner conductor 714. A fluid 725 is circulatedwithin one or both the first and second dielectric materials 716, 718via conduits 727 and 729 defined respectively therein. The relativetemperature of the fluid 725 may be selectively controllable viacircuitry controlled by the generator 300 to regulate thermal expansionof one or both the first and second dielectric materials 716, 718 toposition the inner conductor 714 relative to the outer conductor 712 tochange the impedance of the inner conductor 714. The fluid 725 mayoptionally or alternatively be disposed between the first and seconddielectric materials 716, 718 and be controlled in a similar manner.

While several embodiments of the disclosure have been shown in thedrawings and/or discussed herein, it is not intended that the disclosurebe limited thereto, as it is intended that the disclosure be as broad inscope as the art will allow and that the specification be read likewise.Therefore, the above description should not be construed as limiting,but merely as examples of particular embodiments. Those skilled in theart will envision other modifications within the scope and spirit of theclaims appended hereto.

1-12. (canceled)
 13. A method for controlling alignment of an innerconductor and an outer conductor of a coaxial cable of a microwaveantenna, the method comprising: changing a temperature of at least onethermally responsive material disposed between an inner conductor and anouter conductor of a coaxial cable of a microwave antenna, the innerconductor being disposed within the outer conductor, wherein a change intemperature of the at least one thermally responsive material altersalignment of the inner conductor within the outer conductor.
 14. Themethod according to claim 13, wherein the at least one thermallyresponsive material is a dielectric.
 15. The method according to claim13, wherein changing the temperature of the at least one thermallyresponsive material includes heating the thermally responsive materialvia a plurality of resistive heating elements disposed within thethermally responsive material.
 16. The method according to claim 13,wherein the at least one thermally responsive material includes a shapememory alloy.
 17. The method according to claim 13, wherein the at leastone thermally responsive material includes a plurality of spacersdisposed in a longitudinally spaced apart relationship with respect toeach other.
 18. The method according to claim 13, further comprising:tuning the microwave antenna by altering the alignment of the innerconductor within the outer conductor.
 19. The method according to claim13, wherein tuning further includes repositioning the inner conductorwithin the outer conductor from a first position wherein the innerconductor is concentrically aligned with the outer conductor to a secondposition wherein the inner conductor is not concentrically aligned withthe outer conductor.
 20. The method according to claim 13, furthercomprising: monitoring the position of the inner conductor relative tothe outer conductor.
 21. A method for controlling impedance of a coaxialcable of a microwave probe, the method comprising: measuring impedanceof a coaxial cable, the coaxial cable coupling a microwave probe to amicrowave generator; determining whether the impedance of the coaxialcable substantially matches impedance of at least one of the microwavegenerator or the microwave probe; and changing a temperature of at leastone thermally responsive material disposed between an inner conductorand an outer conductor of a coaxial cable of a microwave antenna basedon the determination, the inner conductor being disposed within theouter conductor, wherein a change in temperature of the at least onethermally responsive material alters alignment of the inner conductorwithin the outer conductor to substantially match the impedance of theat least one of the microwave generator or the microwave probe to thecoaxial cable.
 22. The method according to claim 21, wherein the atleast one thermally responsive material is a dielectric.
 23. The methodaccording to claim 21, wherein changing the temperature of the at leastone thermally responsive material includes heating the thermallyresponsive material via a plurality of resistive heating elementsdisposed within the thermally responsive material.
 24. The methodaccording to claim 21, wherein the at least one thermally responsivematerial includes a shape memory alloy.
 25. The method according toclaim 21, wherein the thermally responsive material includes a pluralityof spacers disposed in a longitudinally spaced apart relationship withrespect to each other.
 26. The method according to claim 21, furthercomprising: tuning the microwave antenna by altering the alignment ofthe inner conductor within the outer conductor.
 27. The method accordingto claim 26, wherein tuning further includes positioning the innerconductor relative to the outer conductor from a first position whereinthe inner conductor is concentrically aligned with the outer conductorto a second position wherein the inner conductor is not concentricallyaligned within the outer conductor.
 28. The method according to claim21, further comprising: monitoring the position of the inner conductorwithin the outer conductor.